Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Cultural ecology wikipedia , lookup
Soundscape ecology wikipedia , lookup
Renewable resource wikipedia , lookup
Theoretical ecology wikipedia , lookup
Triclocarban wikipedia , lookup
History of wildlife tracking technology wikipedia , lookup
Human impact on the nitrogen cycle wikipedia , lookup
T. C. R. White The Inadequate Environment Nitrogen and the Abundance of Animals With 41 Figures Springer-Verlag Berlin Heidelberg New York London Paris Tokyo Hong Kong Barcelona Budapest Thomas C. R. White Dept. of Crop Protection Waite Agricultural Research Institute Glen Osmond, South Australia 5064 Australia ISBN-13:978-3-642-78301-2 e-ISBN-13:978-3-642-78299-2 DOl: 10.1007/978-3-642-78299-2 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution under the German Copyright Law. © Springer-Verlag Berlin Heidelberg 1993 Softcover reprint of the hardcover 1st edition 1993 The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Production Editor: Renate Mllnzenmayer 1YPesetting: K+V Fotosatz GmbH, 64743 Beerfelden 31/3145-5 4 3 2 1 0 - Printed on acid-free paper To the memory of my two mentors H. G. (Andy) Andrewartha and G. B (Joe) Rawlings Acknowledgements My story is, inevitably, built upon the ideas and research of others from Darwin to Dawkins - as reflected in my references. However, there are some whose influence has been general rather than specific, others of whose direct influence I am no longer conscious; still others whose relevant work I have never encountered. To all these I apologize for failing to acknowledge them. The basic outline of my thesis finally crystallized while I was teaching ecology at the University of the South Pacific in Fiji, and it was Hermann Remmert's enthusiastic badgering which started me putting it into book form. But I was only able to properly address the task after retirement, and with the support of an honorary research fellowship at the Waite Agricultural Research Institute. For many years I worked where ready access to the literature was limited. lSI's Current Contents, and the hundreds of reprints sent to me by colleagues throughout the world enabled me to keep up. Too many colleagues to list lent photographs to illustrate the book - mostly colour slides. Jennie Groom did a grand job producing black and white prints from the latter. Many of these same colleagues, and several others, also helped with comments on appropriate sections. Tom Browning read much of an early draft, alerting me to grammatical slips and lapses into teleology. But I continue to deliberately split an occasional infinitive, and resort to teleological analogy if I think this makes for clearer exposition! However, it has been Piet den Boer, with his detailed and wise criticisms via our lengthy discussions by post, who has done more than anybody else to improve both what I have to say, and the way in which I say it. I am especially grateful to him. To all, my very sincere thanks, coupled with the usual exoneration from any responsibility for what I have said. And in this age of word processors I can blame nobody but myself for typing and spelling errors! Last, my very special thanks to my wife, Janice, for grammatical advice, painstaking assistance with final much valuable proof-reading, and, above all, for continuing to believe in the worth of the project, and for keeping me at it on the numerous occasions when I lost my confidence. Preface This is not a text book. Nor is it an objective review. It is a personal - and thus biased - view of some facets of ecology; a view I vigorously advocate. More specifically, it is an attempt to report patterns that I perceive threaded through the fabric of the living world, the common cause of those patterns, and some of the implications for our understanding of ecology that flow from this perception. As a result I come to conclusions that sometimes differ from much that is generally accepted, and with which many will not agree. But I have not felt obliged to put the case for alternative hypotheses or interpretations. I leave those who support them to expound their own viewpoints. In advocating my viewpoint I have taken examples from studies that cover a wide geographic and taxonomic range. However, I have not attempted to be comprehensive. Such would have been a futile task, even had I the energy, or time left, to try to do so. Every story I follow up uncovers several new ones. Yet there remains a majority of the literature that I have never seen - and never will. Apart from this, however, there is simply not room for all the examples that I have found. If I reported these, the book would be double its size, so I have had to knowingly put much good evidence on one side. For example, I have left out major groups like the mites and the nematodes, and the sawflies and leafbeetles among the insects. Some, like the bacteria, get inadequate coverage; others, like the bark beetles, get but passing mention. And within the groups from which I have taken examples, there are many more cases that I could equally have reported. Furthermore, I have not tried to be even-handed in selecting which out of so many examples to use. Rather, I have tried to select studies which tell a reasonably complete story, or which make a particular point unambiguously. For all these reasons what I have to say is incomplete and patchy, but I hope that what I have chosen illustrates the generalities sufficiently, and that others will fill in the gaps. In attempting to write clearly and simply I have eliminated acronyms, all but common abbreviations, and as much jargon, technical language, and quantitative detail as possible. I have balanced the latter, however, with a very full bibliography wherein the reader who so wishes can find these details. x Preface One of the temptations in modern ecological research is to retreat within the protective jungle of complexity. For some, the natural world seems so diverse, and interactions within it so varied, that they believe we cannot expect to find general patterns there. Unlike physics, they say, there can be no hard laws of ecology that will encompass the ever-growing diversity of life; every case will be different. Many delight in this diversity, and present evidence for it in detailed descriptions of the structure of, and interactions within, ecosystems, communities, food webs, and populations. However, I believe that the task of ecologists is to do more than describe - no matter how precisely and quantitatively. Like other scientists we must search for rules that are more general than others, and which explain the complexity and variability we observe. This book is my attempt to contribute to that process. My colleague Piet den Boer warns me, however, that we cannot make science by piling up selected examples. I agree. But by presenting a broad cross-section of examples I can best demonstrate the existence of general patterns in nature. A knowledge of these patterns allows the derivation of new, testable hypotheses about their cause. These, in turn, will lead to the making of more science. This is especially so if they are primary patterns. Feeding on flush tissues, coprophagy, or the early death of most young, for example, are some of the primary ecological patterns that I perceive to be reflecting the common scarcity of nitrogenous food. They are analogous to primary evolutionary patterns, like the skeletal structure of vertebrates, which reflect a common ancestry. Understanding them may also help interpret secondary ecological patterns, like those found in the interrelations within communities and food webs, or in guilds of animals associated with vegetational succession. Others will protest that there are many exceptions which I have not presented. But, as Peter Price (1991a) recently pointed out, exceptions cannot destroy the existence of a pattern - they cannot disprove it. This can be done only when the preponderance of evidence shows that another pattern - or no pattern - exists. Furthermore, many exceptions, when more carefully investigated, prove not to be exceptions at all. They are either reconcilable with the rule they seemed to contradict, or they lie outside of it. There are two basic concepts that underpin what I have to say: the oneness of life; and the importance of the individual. And I do not use either expression in any emotional or anthropomorphic sense; quite the reverse. The enormous diversity of form, function, and interaction of life that we observe today is all part of one pullulating, mindless continuum; the whole evolved from the same simple, early replicators, and programmed and driven by their descendent genes to produce even more genes in the future. [Dawkins (1986, 1989) says all this Preface XI much better than I). Natural selection operates upon the consequent "oversupply" of individual phenotypes, eliminating all that cannot cope. The death of so many is not "wasteful", nor cause for sentimental concern. Nature has no "thought" for the "welfare" of future generations. Natural selection, while determining the future, is yet a process for the moment. Because this is the way nature works, the individual phenotype should be paramount in ecological studies. The environment of an organism is everything that impinges upon it, including those of its own kind (Andrewartha 1970). As Einstein is purported to have said, "The environment is everything that is not me". All of nature comprises individual phenotypes each struggling to survive in its own indifferently harsh environment. It is essential that we think about ecological interactions from the point of view of that individual's struggle. All that said, this book is mostly about herbivores. Not because they are more "important" than carnivores, but because it is not widely recognized or accepted, as it is for carnivores, that herbivores are limited by their food - even less that it is nitrogen, not energy, which is in critically short supply for them. This is the heart of my thesis. A lack of access to nitrogen in a form that can be used for the production and growth of young is the major restriction on the abundance of all animals. This shortage is reflected in a host of common structural, physiological, and behavioural adaptations, which all serve the same function of helping alleviate this "universal nitrogen hunger" (Keeble 1910). Many years ago my friend and colleague Alan Newsome, who has worked all his life with mammals, told me that, for him, insects might just as well be Men from Mars! Mindful that there are many ecologists like him, I have arranged and subdivided the text so that those who wish to find out what I have to say can do so without reading about very much other than "their" animals. But I would plead with them not to stop there. I think they will find that animals which they have never encountered, or have had little to do with since their undergraduate days have, ecologically, much in common with the animals that they work with every day. Then, if I have been half-way successful in gathering and presenting the evidence, they will begin to see the generality of my thesis. Finally, I did not write this book solely for my fellow ecologists. I hope that many others will find what I have to say both useful and enjoyable to read. And students, too, both postgraduate and undergraduate - those who are not yet set in their thinking like we older ones. To them I say: read this, think about it, and then go and look at the world from this point of view. The evidence is there. Adelaide, November 1992 T. c. R. White Contents Part I: The Inadequate Environment Introduction .......................................... 3 Chapter 1. The Environment of All Organisms Is Inadequate ......................................... 5 1.1 Natural Selection Is a Negative Process ............... 6 1.2 Populations Press Against Limits of a Minimum Resource... .. .... . . .. .... . .... .... .... ..... ...... . 1.3 What Essential Resource Is Most Likely to Be Limiting? 1.3.1 Nitrogen the Most Limiting Chemical ........... 1.3.2 Nitrogen Limiting Plants ...................... 1.3.3 Nitrogen Limiting Animals ..................... 1.3.4 Energy Not Limiting .......................... 1.4 Competition a Consequence Not a Cause ............. 1.4.1 Intra-specific Competition ..................... 1.4.2 Inter-specific Competition and "Competitive Exclusion" ................................... 1.5 Self-regulation Does Not Exist ... . . . . . . . . . . . . . . . . . . . . 16 20 Chapter 2. Plants as Food for Herbivores ................ 22 2.1 Why Is the World Green? ........................... 2.2 How Might Plants Be an Inadequate Source of Food? .. 2.3 How and When Might Nutrients in Plants Be Too Dilute? .................................... 2.4 When Is a Minimum Supply of Nitrogen Critically Important? ............................... 2.5 How Might Herbivores Counter the Plants' Evolved Strategies? ........................................ 2.6 How General Is Dilution of Nitrogen in Herbivore Food, and What Adaptations Have Evolved to Counter it? ... 22 23 8 11 11 12 13 14 14 14 23 24 24 26 XIV Contents Part II: Herbivores in an Inadequate Environment Chapter 3. Insects ..................................... 31 3.1 Flush and Senesence Feeders ........................ 3.1.1 Two Australian Psyllids on Eucalyptus .......... 3.1.2 Two African Scale Insects on Californian Ice Plants ...................................... 3.1.3 Two Aphids on Scots Pine .................... 3.1.4 The Green Spruce Aphid in Scotland. . . . . . . . . . . 3.1.5 Aphids on Sycamore, Apple, Wheat, and Alfalfa 3.1.6 Scale Insects on Euphorbia and Euonymus ...... 3.1.7 A Leafhopper with Alternate Generations on Brambles and Oak ........................ 3.1.8 Two Species of Caterpillars Eating Oak Leaves .. 3.1.9 Two Species of Sawflies Mining in Birch Leaves. 3.1.10 A Chewer and a Sucker on Poplar Leaves. . . . . . . 3.2 Leaf-miners ....................................... 3.2.1 The Switch from Flush to Senescence Feeding ... 3.2.2 Leaf-miners Which Induce "Green Islands" in Leaves ................................... 3.2.3 A New Zealand Weevil Mining in Fallen Beech Leaves ................................ 3.2.4 The American Holly Leaf-miner ............... 3.3 Gall Makers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Physiological Galls ........................... 3.3.2 Nutritional Benefits of Galling ................ 3.3.3 Adaptive Nature of Galls Debated Anew ........ 3.3.4 Double-dipping: Prolonged Growth plus Hastened Senescence .................................. 3.3.5 Selection for High Nitrogen and Survival of the Young ................................ 3.3.6 Selection of Growing Tissues for Proliferation of Galls .................................... 3.4 Chewing Insects ................................... 3.4.1 Creaming-off as a Tactic to Increase Access to Nitrogen: White Butterflies on Crucifers ..... 3.4.2 Early Instars Need More Nitrogen: Gypsy Moth on Artificial Diet ............................ 3.4.3 Illustrations from the Life Cycles of Economically Unimportant Butterflies ...................... 3.4.4 Further Examples from Forest Defoliators ....... 3.4.5 Pests of Crops also Reveal the Need for Nitrogen 3.4.6 Examples from Biological Control of Weeds .... 3.4.7 Locusts and Grasshoppers .................... 3.5 Sap-Sucking Insects ................................ 32 34 37 38 39 41 42 43 44 45 46 48 48 49 49 50 52 53 54 55 57 58 61 62 63 65 65 70 75 77 80 85 Contents XV 3.5.1 Aphids ...................................... 3.5.2 Psyllids ...................................... 3.5.3 Scale Insects ................................. 3.5.4 Planthoppers, Leafhoppers, and a Mirid ......... 3.5.5 Xylem-feeders ................................ 3.6 Fruit Flies ........................................ 3.7 Wood-eating Insects ................................ 3.7.1 The Key Role of Fungi: Increasing Nitrogen in Wood..................................... 3.7.2 Termites and Woodroaches: Gut Fauna, Coprophagy, and Recycled Nitrogen ............. 3.7.3 Furniture and Longhorn Beetles ................ 3.7.4 Woodwasps .................................. 3.7.5 Borers Which Do Not Ingest Wood ............. 85 89 92 93 95 97 98 100 104 106 106 Chapter 4. Crustaceans ................................ 108 4.1 Microcrustaceans .................................. 4.1.1 Distribution and Abundance of Food Limited by Nitrogen .................................. 4.1.2 Nitrogen Content of Food also Important ....... 4.1.3 Microcrustaceans Feed Selectively for Nitrogen ... 4.1.4 Coprorhexy .................................. 4.2 Macrocrustaceans: Land Crabs, Lobsters, and Shrimps . 4.3 Terrestrial Crustaceans - The Isopods ............... 4.3.1 The Case of a Common Woodlouse............. 4.3.2 The Role of Microorganisms and Coprophagy .... 108 108 110 112 115 116 117 117 119 Chapter 5. Molluscs ................................... 123 5.1 Some Examples of Freshwater Snails ................. 5.2 Marine Limpets and the Flow of Nitrogen Through the Food Chain ........................... 5.3 Death of the Young, Selective Feeding, and Animal Protein in the Diet ................................. 5.4 Detritus Feeders Feed Selectively, and Depend upon Microorganisms and Coprophagy .................... 5.5 Terrestrial Snails Live with the Same Constraints ...... 5.6 Cannibalism by Young Snails, Illustrates Shortage of Nitrogen ....................................... 5.7 Teredo Shipworms Depend upon Microorganisms Which Fix Atmospheric Nitrogen .................... 123 99 125 127 129 130 131 133 Chapter 6. Mammals .................................. 135 6.1 Large Mammals ................................... 6.1.1 Feral Donkeys in Australia ..................... 135 136 XVI 6.2 6.3 6.4 6.5 Contents 6.1.2 Red Deer in Scotland 6.1.3 Antelope, Giraffe, and Greater Kudu in Africa .. . 6.1.4 Deer in North America ....................... . 6.1.5 The Giant Panda in China .................... . 6.1.6 Domestic Stock .............................. . Rodents ......................................... . 6.2.1 Squirrels .................................... . 6.2.1.1 True Squirrels ......................... . 6.2.1.2 Chipmunks and Ground Squirrels ....... . 6.2.2 Rats and Mice ............................... . 6.2.2.1 The House Mouse ..................... . 6.2.2.2 The Australian Smokey Mouse .......... . 6.2.2.3 The American White-Footed Mouse ..... . 6.2.2.4 American Woodrats ................... . 6.2.3 Voles ....................................... . 6.2.4 Supplemental Feeding of Small Rodents ........ . 6.2.5 Rabbits and Hares ........................... . 6.2.5.1 The European Mountain Hare .......... . 6.2.5.2 The European Rabbit .................. . 6.2.5.3 The North American Snowshoe Hare .... . Primates ......................................... . 6.3.1 Colobine Monkeys ........................... . 6.3.2 Cercopithecid Monkeys ....................... . 6.3.3 Howler Monkeys ............................. . 6.3.4 The Gorilla ................................. . Fruit and Flower Bats ............................. . Marsupials ....................................... . 6.5.1 The Koala .................................. . 6.5.2 Possums and Gliders ......................... . 6.5.3 The Habitat of Possums and Gliders ........... . 6.5.4 Kangaroos and Wallabies ..................... . 137 138 141 142 144 144 144 145 148 150 151 154 155 156 156 162 164 165 166 167 172 172 174 174 175 178 181 181 184 187 189 Chapter 7. Birds ..................................... . 193 7.1 Birds Eating Green Leaves ......................... . 7.1.1 Geese in Europe and North America ........... . 7.1.2 European Grouse, Ptarmigan, and Capercaillie .. . 7.1.3 North American Grouse ...................... . 7.1.4 Partridges and Pheasants ..................... . 7.1.5 Galliforms as Hindgut Fermenters ............. . 7.1.6 Changes in Abundance of Lagopus Species ..... . 7.1.7 The Thkahe ................................. . 7.1.8 The Hoatzin ................................ . 7.2 Birds Eating Nectar and Fruit ...................... . 7.3 Birds Eating Seeds ................................ . 7.3.1 Columbids .................................. . 193 193 200 207 210 211 212 215 217 217 221 221 Contents XVII 7.3.2 African Queleas, European Finches, and the Great Tit ........................... 7.3.3 Darwin's Galapagos Finches ................. 7.3.4 The Australian Galah ....................... 222 226 230 Chapter 8. Reptiles .................................... 233 8.1 The Giant Tortoises of Aldabra Atoll.. . . .. . . .. .. . . .. 8.2 The Green Turtle of the Bahamas Islands ............ 8.3 The Marine and Terrestrial Iguanids of the Galapagos Islands .......................................... 8.4 The Desert Iguanid of California ................... 8.5 The Green Iguanid of Panama ..................... 233 236 Chapter 9. Fish ....................................... 242 9.1 9.2 9.3 9.4 242 244 245 247 The Carnivorous Young of Fish .................... Fish Which Eat Detritus ........................... Fish Which Eat Algae ............................. Gut Microbes and Coprophagy in Fish .............. 237 239 240 Part III: Survival in an Inadequate Environment Chapter 10. Strategies to Counter Shortage of Nitrogen 10.1 Strategy A: Synchronize the Life Cycle with Availability of Good Food .................................... 10.2 Strategy B: Concentrate or Prolong Availability of Nitrogen in Food ............................... 10.3 Strategy C: Eat More Food More Quickly, and Digest More Efficiently ........................ 10.4 Strategy D: Enlist the Help of Microorganisms ....... 10.5 Strategy E: Supplement Plant Food with Animal Protein .............................. 10.6 Strategy F: Apportion and Concentrate the Limited Food to a Select Few .............................. 253 253 254 255 256 258 260 Chapter 11. Territorial and Social Behaviours ............. 261 11.1 Territorial Behaviour in Carnivores .................. 11.1.1 Birds ...................................... 11.1.2 Lizards .................................... 11.1.3 Insects .................................... 11.1.4 Spiders .................................... 11.2 Territorial Behaviour in Herbivores .................. 11.2.1 Mammals .................................. 264 264 265 266 267 268 269 XVIII Contents . . . . . 271 274 278 280 281 287 Chapter 12. Cannibalism .............................. . 291 12.1 Cannibalism by Females Producing Young .......... . 12.2 Cannibalism by Growing Young ................... . 12.3 Cannibalism, Warfare, and Protein ................. . 292 295 297 11.2.2 Birds 11.2.3 Insects ................................... 11.2.4 Fish ...................................... 11.2.5 Molluscs .................................. 11.3 Surplus Young, Dispersal, and Philo patry ........... 11.4 Social Structures and Dominance Hierarchies ........ Part IV: Predators in an Inadequate Environment Chapter 13. Vertebrates .................................. 304 13.1 13.2 13.3 13.4 Lions, Lynx, and Feral Cats Coyotes, Wolves, and Foxes Stoats, Mice, and Seed Mast Pelicans, Puffins, and Other ........................ ........................ ....................... Sea Birds. . . . . . . . . . . . . .. 304 306 307 308 Chapter 14. Invertebrates ............................... 314 14.1 14.2 14.3 14.4 14.5 314 315 317 319 323 Triclad Worms ....................... , . . .. . ... .... Spiders and Scorpions ............................. Ground Beetles, Tiger Beetles, and Ant-lions ......... Praying Mantids .................................. Parasitoids, Parasites, and Diseases .................. Part V: The Alleviation of an Inadequate Environment: Outbreaks Chapter 15. What is an Outbreak ......................... 333 15.1 Some Examples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 15.2 What Causes Outbreaks? .......................... 15.3 The Paradox of Enrichment and "r" and "K" Strategists ........................................ 333 336 338 Chapter 16. The Interaction of Food, Prey, and Predators in Outbreaks ............................ 341 16.1 Bacteria and Protozoa ............................. 16.2 Rabbits, Foxes, Cats, and Dingoes .................. 16.3 The Varying Response of Predators to Changes in Prey 341 342 343 Contents XIX 16.4 A Natural Experiment: Guano-Algae-LimpetsOystercatchers .................................... 16.5 A Thought Experiment: Hot Spots in a Box of Wadding ...................................... 347 Chapter 17. Cyclic Outbreaks ........................... 350 Chapter 18. The Influence of Weather on the Generation of Outbreaks ......................................... 356 18.1 18.2 18.3 18.4 18.5 18.6 345 Hot Spots Again: Outbreak Centres and Boundaries .. Spruce Budworm Outbreaks Revisited ............... Patchy Environments and Metapopulations ........... The Role of Viral Diseases ......................... The Link to Climatic Oscillations ................... Major Outbreaks Which Are Independent of the Weather ................................... 357 359 361 362 363 References ............................................ 367 Subject Index ......................................... 413 365 Part I: The Inadequate Environment Introduction Over 80 years ago the Professor of Botany at the then University College of Reading in England, Frederick Keeble, published a small book (Keeble 1910). I discovered this book in 1985! In it he described his simple but elegant and imaginative experiments with the green and the yellow-brown marine platyhelminth worms Convoluta roscojjensis and C. paradoxa. He demonstrated, inter alia, that the relationship between these worms and their symbiotic photosynthetic algae is driven by the lack of available nitrogen in the environment. The algae are mostly free-living photosynthetic flagellates. They exist precariously in an environment notoriously deficient in nitrogen, but some become embedded in the mucilagenous egg capsules of the worms. There, with access to a much improved supply of nitrogen, they rapidly multiply. Some are ingested by the newly hatched worms, and divide repeatedly to form a dense mass of photosynthetic cells throughout the worms' bodies, using the metabolic waste of the worms as their source of nitrogen (the worms have no excretory system and, in the experimental absence of algae, accumulate vacuoles full of crystals of nitrogenous waste). The worms are colourless when they hatch and feed by ingesting single-celled plants and animals in the water. Once their contained algae have multiplied and started to photosynthesize they reduce, or in the case of C. roscojjensis, cease their feeding. They continue to grow, and eventually produce eggs, using the products of algal photosynthesis for their nutrition. (When artificially prevented from acquiring the algae they whither and die.) After the production of eggs, however, in this closed system of exchange of nitrogen back and forth between worm and alga, nitrogen again becomes limiting. The worms then digest their algae before themselves dying. Keeble pointed out that to label the association of worm and alga as symbiosis is to miss the significance of the association. For the worm it is obligate dependence upon the alga - parasitism. Without the alga it can neither rid itself of nitrogenous wastes nor gain further nutrition. For the alga, on the other hand, that some few of its individuals enter this association with a worm is "... an episode without significance.. ?'. Most of them remain free as green photo synthesizers or colourless saprophytes "...which batten on the offal of the sea .. ?'. But for an ingested cell the association with the animal solves the problem of acquiring enough nitrogen. As Keeble so colourfully expressed it, "It sacrifices its independence for a life of plenty. This universal nitrogen-hunger is a misery which makes strange bedfellows?' . 4 Introduction Indeed it does. It is a misery which drives the ecology of all organisms, and Keeble was at pains to emphasize this fact. "The nitrogen problem" as he called it, "... stems from the shortage of nitrogen which is available for the formation of nitrogenous food for plants", and as such is, "...the problem which besets all living organisms". This nitrogen hunger, he said, is "... no small matter of mere academic importance". It " ... presents a problem which every living organism must solve. The supply of available nitrogen is a limiting factor of life" (my emphases). An awareness of the fundamental role that this limitation of nitrogen plays in the ecology of all organisms should be as much a part of each ecologist's intellectual equipment as is the awareness of the fact of evolution by means of natural selection. What follows is my attempt, (having painfully reinvented the wheel!), to get this message across to future ecologists. Please do not let it languish for another 80 years!. Chapter 1 The Environment of All Organisms Is Inadequate This chapter title seems a fairly obvious statement to make about a world in which only a small fraction of the offspring of all organisms survive to pass on their genes to the next generation. The world must be a harsh and inhospitable place for this to happen. Yet there still seems to be plenty of space, and organisms rarely use more than a fraction of the resources in their habitats. Much of conventional ecology says that this is because there are many and varied environmental factors, and numerous competitive and social interactions, which regulate numbers of organisms below those which their resources would allow; which contain the innate tendency of all organisms to increase progressively. The environment is therefore not limiting or inadequate. Exhaustion of a resource, probably food, would ultimately limit further increase of any organism, but this does not happen - or happens only rarely when the regulators temporarily cease to operate. For most of the time there are processes operating which keep populations in balance around equilibrium levels well below those which would use up all the resources and destroy the environment. This view has it that the struggle for existence in nature follows remorselessly from the capacity of organisms to increase their numbers exponentially. But rather the reverse is true. The capacity of all organisms to increase their numbers exponentially follows remorselessly from the struggle for existence. Surviving on this earth is, and always has been, especially for the very young, a struggle, a chancy business. The huge "biotic potential" of all organisms is the universal illustration of that fact. The capacity to increase exponentially did not evolve to provide a struggle for existence as a vehicle for evolution. Nor need it be espoused as the reason why populations must be regulated. It evolved because, only in populations in which females produced many offspring did sufficient individuals gain access to sufficient resources, and survive in each generation, to enable the population to persist. How did this come about? Because this is a finite world, and in a finite world there was one sure consequence of the evolution of the first self-replicating entities. Sooner or later a resource, essential to their growth and replication, ran out. There was no longer enough for all seeking to use it. No longer could all replicate. No longer were all potentially immortal; differential survival had arrived. From the moment a resource became limiting the environment became less adequate, and natural selection began to operate. The struggle for existence - to gain enough of that resource to replicate - had begun. The "lucky" few that happened to get enough replicated; those that failed perished. Then, as now, anything which lessened the chance of an individual being one 6 1 The Environment of All Organisms Is Inadequate of those few ensured the eventual elimination of that individual's genes from the gene pool. All else in the living world has followed from this. In such a world balance is an unnecessary idea. The usual and variable shortage of a resource provides an alternative explanation. The tacit assumption in the balance of nature is that all species of organisms tend to produce an abundance of progeny that would survive to reproduce and lead to ever increasing numbers unless controlled by negative feedback mechanisms. As a consequence the mean observable density over a number of generations is seen as an optimum or equilibrium density. The further the numbers depart from this mean the greater the density-dependent pressures in the system pressing them back toward this mean. However, there is no such thing as a mean population in nature. The mean is merely a statistic - an arbitrary, albeit frequently useful, abstraction - derived from a series of samples of the numbers in a population in which numbers are continually changing. These numbers are not deviations from a predetermined mean; they define it. They result from the members of the population struggling, generation after generation, to exploit the limited opportunities afforded by the inadequate environment. Nor is there an "optimum" or "equilibrium" density of a population in nature - only the maximum number that can survive each generation in a population that is pressing hard against the variable but limited supply of resources in its environment. To use the original engineering analogy of steam generated by heat being released through a negative feedback valve, ecologists should be looking to see what generates the steam, not how much steam passes through the regulating valve. In nature rarely is enough steam generated to make the valve operate. I (White 1978) and others before (Den Boer 1968) and since (Dempster and Pollard 1981; Den Boer 1987) have made these points, but the arguments go on (Wolda 1991)! 1.1 Natural Selection Is a Negative Process The role of natural selection in deciding which individuals survive does not involve positive, active selection of those lucky few. It is a negative, passive process which eliminates all the rest; in Den Boer's (1985) words "the non-survival of the non-fit". The environment is inadequate to support all those seeking to live in it, so most will fail to do so. They will be selected out or selected against. Wallace (1866) made this very point in a letter to Darwin. He was urging him to adopt Spencer's term "the survival of the fittest" in place of "natural selection", and expressed concern at the misunderstanding of the latter, and the constant "...comparing it in its effects to man's selection.. !'. "Nature", he said, "does not so much select variations as exterminate the most unfavourable ones". Some years ago I made the same point to a colleague. His response was "So what? The glass is half empty or half full!" But I think it is important that we do make the distinction, for two reasons.